Discrete light sources such as light-emitting diodes (LEDs) are an attractive alternative to incandescent light bulbs in illumination devices due to their higher efficiency, smaller form factor, longer lifetime, and enhanced mechanical robustness. However, the high cost of LEDs and associated heat-sinking and thermal-management systems have limited the widespread utilization of LEDs, particularly in general lighting applications.
The high cost of LED-based lighting systems has several contributors. LEDs are typically encased in a package and multiple packaged LEDs are used in each lighting system to achieve the required light intensity. In order to reduce costs, LED manufacturers have developed high-power LEDs, which can emit relatively higher light intensities by operating at relatively higher currents. While reducing the package count, these LEDs require relatively higher-cost packages to accommodate the higher current levels and to manage the significantly higher heat levels that result. The higher heat loads and currents, in turn, require more expensive thermal-management and heat-sinking systems—for example, thermal slugs in the package, ceramic or metal submounts, large metal or ceramic heat sinks, metal core printed circuit boards and the like—which also add to the cost as well as to size of the system. Higher operating temperatures also lead to shorter lifetimes as well as reduced reliability. Finally, LED efficacy typically decreases with increasing drive current, so operation of LEDs at relatively higher currents results in a relative reduction in efficacy as compared with lower-current operation. In order to support high-current operation, the LED chip (inside the package) requires relatively larger contact areas. In addition, high-power LEDs often have a current-blocking layer under the contacts to prevent light emission in those areas. The larger contact areas and current-blocking layer diminish the light-emitting area of the chip, resulting in reduced efficiency, fewer chips per wafer and increased cost.
Contact size is further limited by the method used to connect the LED chip to the package, another substrate or other supporting components. Most commonly, LED chips are interconnected using wire bonding. The wire-bonding process requires a certain minimum contact area, independent of current level. As a result, even in low-current LEDs, the contact size cannot be reduced below the minimum required for wire bonding. Another common approach for connection of the LED chip to the package is to use agents such as solder or conductive adhesives to bond a LED to a package, submount or substrate. These agents may also be relatively expensive and require complicated processes to control their dispersion so as to prevent the contacts of the LED from shorting together and rendering the device inoperative; this is particularly so as device geometries (for example, spacing between contacts) and dimensions continue to shrink.
One recent advance facilitating the connectivity of LEDs to a variety of substrates is anisotropically conductive adhesive (ACA), which enables electrical interconnection in one direction (e.g., vertically between a device contact and a substrate contact), but prevents it in other directions (e.g., horizontally between contacts on a device or between contracts on a substrate). State-of-the-art ACAs are pressure-activated, and thus require provision of “stud bumps” or other metallic projections on the surface to which the LED is to be bonded or on the LED bond pads in order to create the anisotropic electrical conductivity and promote adhesion. While other, non-pressure-activated types of ACA exist (e.g., ZTACH available from SunRay Scientific of Mt. Laurel, N.J., for which a magnetic field rather than pressure is applied during curing in order to align magnetic and conductive “columns” in the desired conduction direction), such ACAs are less common and require additional, and potentially expensive, equipment (e.g., magnets).
As known in the art, a pressure-activated ACA typically comprises an adhesive base. e.g., an adhesive or epoxy material, containing “particles” (e.g., spheres) of a conductive material or of an insulating material coated with a conductive material (such as metal or a conductive material coated with an insulating material. FIG. 1 depicts a conventional use of pressure-activated ACA to connect an electronic device to a substrate. As shown, the electronic device 100 having multiple contacts 110 has been adhered and electrically connected to a substrate 120 via use of an ACA 130. The ACA 130 comprises an adhesive base 140 containing a dispersion of particles 150 that are at least partially conductive. As mentioned above and as shown in FIG. 1, conventionally, the use of ACA requires the target substrate to contain stud bumps (which typically have a thickness of at least 30 μm-50 μm), or other conductive structures projecting from the substrate, opposite the device contacts to be bonded in order to achieve adequate bonding and electrical connectivity between the device and the electrical interconnects on the substrate. That is, in the context of FIG. 1, the adhesion and electrical connection of contacts 110 to electrical traces 160 (the thickness of which has been exaggerated for clarity) on substrate 120 requires the presence of stud bumps 170. As shown, the conductive particles 150 provide electrical connectivity between each contact 110 and its respective trace 160, but are dispersed within base 140 at a sufficiently low density such that an electrical connection is not formed between the contacts 110 and/or the traces 160. The stud bumps 170 provide not only a portion of the electrical connection, but also a solid platform against which the particles 150 are compressed, sharply increasing the conductivity of the ACA 130 and enabling the electrical connection therethrough (but not across the uncompressed ACA between the contact/stud bump pairs). In an alternate geometry, the stud bumps may be attached to contacts 110. It should be noted that other techniques involving ACAs are possible, and the present invention is not limited by the particular mode of operation of the ACA.
However, the use of stud bumps or equivalent conductive structures may be problematic and costly in many applications. Particularly as device and device-contact dimensions continue to decrease, stud bumps are frequently too large for connection to individual contacts. Formation of stud bumps also necessarily entails the formation of topography on the substrate, a complicated and expensive process, particularly when device contacts are non-coplanar (as stud bumps of a variety of heights are required). Furthermore, in applications utilizing unpackaged semiconductor die (e.g., bare-die LEDs), bonding of the device to stud bumps may result in deleterious localized stress (e.g., if the die bows between stud bumps due to the applied bonding pressure). Finally, use of stud bumps or similar structures may result in thermal-expansion mismatch (and concomitant stress) between the bumps and the substrate or bonded die.
However, without stud bumps or other projecting structures, bonding a semiconductor die to conventional substrates will not result in a reliable electrical connection therebetween, particularly if the contacts on the semiconductor die are non-coplanar. FIG. 2 depicts a common device environment that illustrates the problem. As shown, a LED die 200 features a contact 210 to an n-doped layer 220 and a contact 230 to a p-doped layer 240. A portion of the p-doped layer 240 has been removed to enable formation of contact 210 over the n-doped layer 220, rendering contacts 210 and 230 non-coplanar. In FIG. 2, an attempt has been made to bond LED die 200 to a conventional substrate 120 (e.g., a printed circuit board), which is substantially rigid and non-deformable. Due at least in part to the non-coplanarity between contact 210 and contact 230, the particles 150 of the pressure-activated ACA 130 establish electrical contact in the compression zone between contact 230 and its corresponding trace 160-1, but, in the absence of stud bumps, a similar electrical connection cannot be formed between contact 210 and its corresponding trace 160-2 due to the absence of sufficient compression. Even if a temporary electrical connection is initially formed between contact 210 and trace 160-2, upon cure of the ACA 130 and/or during operation, the ACA 130 may expand or contract, resulting in the loss of electrical contact and inoperability of LED die 200. Such expansion and/or contraction may also occur during operation, for example from ambient heating or self-heating from operation, resulting in unreliable operation.
In view of the foregoing, a need exists for systems and procedures enabling the low cost reliable bonding of various semiconductor dies (e.g., LED dies and solar cell dies) directly to a substrate's electrical traces via pressure-activated adhesives without the use of stud bumps or similar structures and low cost, reliable LED-based lighting systems based on such systems and processes.